In performing work operations on a tubular workpiece, it is often necessary to firmly hold or clamp the workpiece in position. For example, when a jeweler works on a finger ring, it is necessary to firmly mount the ring in position. Firm mounting of the ring is essential because of the very precise nature of the jeweler's tasks. This ostensibly simple task is actually very difficult because finger rings are typically delicate and made of soft metals. For these reasons, it is very difficult to firmly clamp such a finger ring without deforming or disfiguring it.
There are a number of different prior art devices for mounting finger rings. One such device includes a tapered mandrel on which the ring is mounted. However, the tapered mandrel does not hold the ring steady while the jeweler is working on the ring.
U.S. Pat. No. 2,707,107 issued to Tucker and U.S. Pat. No. 3,866,494 issued to Dotson show devices for holding rings which comprise multiple fingers movable radially outwardly to grip the internal surface of the ring. These fingers engage a relatively small area of the ring, and consequently, the unit loading; i.e., stress, on the portions of the ring so engaged must be relatively high if the ring is to be firmly held in position. This high unit loading tends to deform and/or disfigure the ring. The deformation may result in increasing the inner diameter of the ring so that it is necessary to size the ring.
The patented literature also discloses devices which are adapted to clamp other kinds of tubular articles. For example, Jackson U.S. Pat. No. 2,850,287, Czerenda U.S. Pat. No. 2,929,635 and Gross, et al U.S. Pat. No. 3,776,562 are in this group. The latter two patents of this group are directed to apparatuses which include an expandable collet for holding a tubular workpiece. The workpiece is slipped on the expandable collet, and the collet is expanded to clamp the workpiece. The range over which a collet can be radially expanded is limited and so a wide range of collets must be provided in order to accommodate rings or other workpieces of different internal diameters. Changing collets is a relatively time consuming task. In addition, it is relatively expensive to provide the necessary number of collets.